Electro-optical image production



Aug. 25, 1942. c. L. wEls, ,IR 2,293,870

ELECTRO OPTICAL IMAGE PRODUCTION Filed Feb. s, 1940 s sheets-sheet 1 BV V A TTOR/VEV Aug. 25, 1942.

c. L, wels, JR 2,293,870 ELECTRO OPTICAL IMAGE PRODUCTION Filed Feb. 3, 1940 3 Sheets-Sheet 2 Magna/wb A7' TORNEV Aug. 25, 1942. c, wEls, JR l ELECTRO OPTICAL IMAGE -PRDUCTION Filed Feb, 5, 1940 3 Sheets-Sheet 3 hun llAlll` /VVENTOR @y LWE/s, JR.

ATTORNEY Patented Aug. 25, 1942 2,293,870 o ELECTRO-OPTICAL IMAGE PRODUCTION Charles L. Weis, Jr., Mount Vernon, N.

Y., assignor to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application February 3, 1940, Serial No. 317,104

g 2 Claims. (Cl. TIS-5.6) This invention relates to electric signaling and more particularly to electro-optical image production.

An object of the invention is to provide a novel method of, and means for, accomplishing single side-band transmission in which use is made of the principle of vestigial side-band operation.

Another object is to provide for effective single side-band transmission of wide frequency band signaling currents over a cable of the coaxial type.

A further object is the effective selection of frequencies for effecting the transmission of a wide frequency signal band on a carrier basis, said transmission being within a range overlapping said signal frequency band.

Other objects and features of the invention' will be apparent as the description of the invention proceeds.

The invention, in its various aspects, may be understood from the following brief description of an embodiment of it herein chosen for purposes of illustration and later described in detail. In this particular embodiment a coaxial cable is employed as the transmisison medium for simultaneously transmitting television, speech and television synchronizing current. The particular range of frequencies employed for each transmission and their spacing in the frequency spectrum were chosen, as explained be- ]oW. Let it be assumed that the coaxial cable is designed to transmit television current of frequencies in the range between about 100 kilocycles and about 1000 kilocycles without undue distortion and cuts off in the region of 60 kilocycles. While a cable may be designed to satisfactorily transmit lower television frequencies, the diculties encountered are such as to increase the cost. The cable when designed to transmit television frequencies satisfactorily only above 100 kilocycles, introduces phase shifts at lower frequencies of such magnitude as to unduly distort the television image. It is also found that in the region immediately below that where phase shifts for television become prohibitive, telephone transmission may be eiTe'cted with tolerable distortion and that although the synchronizing frequency must be transmitted with a certain amount of fidelity the requirements are not so stringent as to preclude the synchronizing frequency or frequencies being transmitted in the region below 100 kilocycles. Therefore, sound signals are transmitted in the region immediately below 100 kilocycles and the synchronizing frequencies just below the range assigned to sound frequencies. For the transmission of television on the basis of 24 frames per second and a resolution of 240 lines, which is here chosen by way of example, the frequency band resulting from the scanning operation extends from zero to about 806 kilocycles. The problem therefore is to transform this wide frequency television band into a band which may be transmitted over the cable in the range between kilocycles and 1000 kilocycles. In accordance with the invention this is accomplished by flrst modulating the wide frequency band upon a` carrier current of such frequency that a. considerable space in the frequency spectrum is left between the upper limit of the original modulating frequency band, which will appear as a modulation product, and the lowermost frequencies of the lower side-band resulting from the modulation. In selecting the frequency of l the carrier higher order modulation components have to be taken into account. 'I'he next step in the process is that of lowering or shifting the modulated carrier frequency to the desired range for transmission, that is, a range from 100 kilocycles to 1000 kilocycles. This is accomplished by selecting a single side-band and modulating it upon a suitable carrier. In the utilization of a single side-band it was desired to employ the so-called principle of vestigia] side-band operation according to which one side-band is selected along with a vestige of the other sideband in the immediate vicinity of the carrier frequency and the selected side-band is so shaped in the region adjacent the carrier frequency corresponding in width to the vestigial side-band that at the receiver the Vestigial side-band components may be made to combine with the complemental frequencies vderived from the transmitted side-band in such a manner as to givev the equivalent effect of transmission on a single side-band having no amplitude distortion in the region of the carrier. This principle of providing a vestigial side-band to enable single sideband transmission is disclosed in the patent to H. Nyquist 1,748,186, patented February 25, 1930. The problem here under consideration, however, greatly complicates the application of the principle of vestigial side-band transmission in that in the application of that principle great care must be used to design a filter the output of` which has in it the complemental frequencies in proper phase as well as amplitude, and proper phase control at the frequencies here involved is most dimcult. After selection of the wanted side-band and the vestigial other side-band the resulting current is appliedto a second modulator along with a second carrier frequency of such value as to step the selected frequencies into the cable transmission range. In accordanceA sion. A frequency for the synchronizing carrier was then selected such that it would fall below and close to the speech band and still be suitable to serve as a base frequency for generating harwith-the invention there is employed a filter for 5 monic frequencies for the first and second telethe selection of the wanted side-band and a ve'stige of the unwanted side-band which does not produce the amplitude and phase characteristics of the complemental frequencies of the two sidebands in a manner to make use of the operation disclosed in the Nyquist patent, thus avoiding the complication of having to design a filter or correcting network to obtain these proper amplitude and phase relations at the high frequencies vision carriers. Taking into consideration all the requirements above stated it was found that these requirements could be met by selecting the synchronizing carrier at 72 kilocycles, impressing l0 this frequen'cy upon a harmonic generator to generate odd harmonics, and selecting the thir- 'ty-third harmonic for the first television carrier `\and the thirty-fifth harmonic for the second television carrier. This put the first television which the filter must pass. The process of obcarrier at 2376 kilocycles, in a position Where taining the correct amplitude and phase relation of the complemental frequencies is deferred until the band is stepped to a lower range in which adequate phase relationships as well as proper proper selection between the wanted and unwanted productsof the first modulation could be made without introducing difiiculties introduced by selecting the carrier at a higher point.

amplitude discrimination may be obtained. It It was found that the thirty-fifth harmonic,

will be seen, however, that the band-pass filter after the first modulator in part accomplishes one step involved in the use of the vestigial sideband, namely, the selection of the necessary fre- .quencies to be used in the application of the vesulation which corresponds to the first carrier .tigial principle. Since it is impossible to design a filter which will cut off sharply it was necessary to place the cut-off where it would select the vestigial in a manner to change the amplitude of having a frequency of 2520 kilocycles fulfilled all the requirements set forth above with respect to the choice of this carrier. The frequency in the lower side-band resulting from the second modfrequency of 2376 kilocycles and the zero component of the original television signal is 144 kilocycles, namely, the dierence between the frequencies of the first and second carriers.

its components or at ahigher point where it This reference frequency of 144 kilocycles falls would leave these components intact and cut off the` remaining portion of the upper side-band, leaving a further undesired vestige. In accord" ance with the invention the latter course is in the television range suitable for transmission over the coaxial cable. The shifted frequencies corresponding to the lower side-band of the 2376- kilocycle carrier modulation lie above this refadopted, the filter being designed to cut off in erence frequence of 144 kilocycles and the vessuch manner, taking into consideration the position of the second carrier frequency, that none of the modulation components of the second modulator resulting from the unwanted vestigial tigial upper side-band selected by the band-pass filter from the first modulation products lie below this reference frequency.

A specially designed modulator in accordance will appear in the television transmission band with this invention is used for the first modurange of 100 kilocycles to 1000 kilocycles. A band filter constructed to perform this filtering operation introduces undesirable phase shifts which can, however, be corrected, along with subsequent phase shifts, after the second step of 4 modulation. in order to obtain the proper phase relations for application of the vestigial sideband principle. In the illustration given a frequency of 2520 kilocycles was chosen for the second carrier so that the selected lower side-band of this modulator.

appeared in the output of the second modulator as side-bands of the second carrier, the lowermost of which extended from about 70 kilocycles to 950 kilocycles, the position of the original carrier in the lower side-band at 144 kilocycles 55 and the extremity of the Wanted vestigial farthest remote from the carrier, at about 100 kilocycles. The corresponding upper side-band extended from about 4089 kilocycles to 4944 kilo- This modulator is disclosed and claimed in Patent 2,273,548, issued February 17, 1942, the application for which was filed September'24, 1940 as a true division of this application.

While .the invention is being described in a single embodiment making use of a concentric cableas the transmitting means, it should be Aunderstood that in certain of its broader aspects the invention is not limited to cable operation cycles. By means of a low-pass filter the lower and that some of the features of the invention side-band was then selected.

One requirement not yet mentioned was taken into account in the selection of the first and second carrier frequencies. In accordance with the are of broader application than to transmission in television.

The invention will now be described more in detail with reference to the accompanying drawinvention it was desired to transmit the speech ings:

in a range just belowthat suitable for television and the synchronizing frequencies in a range just below that suitable for speech. To accomplish this the program and synchronizing fre- Figs. 1 and 2, taken together, illustrate schematically a television system embodying this invention; and

Figs. 3 and 4, taken together, show the circuit quencies were modulated" upon carirers of suitarrangements of the first and second modulators able frequency to place these transmissions in the desired ranges respectively. The speech carrier frequency was chosen at 84 kilocycles. After modulation the lower side-band extending and the band-pass filter therebetween for passing the lower side-band and vestigial upper sideband from the first modulator.

The illustrative embodiment of the invention to about '13 kilocycles was selected for transmis- 75 shown schematically in Figs. 1 .and 2 will nO'w be described. Toillustrate4 the complete system, Fig. 1 should be placed on the left-hand side of Fig. 2 with the lines L joined together. The circuit elements located at the transmitter are shown in the lower portion of Fig. 1 and those located at the receiver are shown in the lower portion of Fig. 2. 'I'he frequency bands present at various points in the circuit and the energy distribution within these bands, sometimes spoken of herein as the shape of the band or the band shape, are shown in the upper portions of Figs. 1 and 2.

'I'he television signal or video signal originates in a light sensitive electric device I, such as a photoelectric cell or a photoelectric electron multiplier, energized by light under the control of suitable scanning devices. For the system being described, a scanning disc provided with a, circle of lenses was used to scan a uniformly moving sound picture film. Each frame was scanned in 240 lines at the rate of 24 frames per second producing a video signal extending from zero frequency to an upper useful frequency of approximately 806 kilocycles. The scanning light energized a photoelectric electron multiplier. to produce the video signals.

The major portion of the video signal band is'- shown by cross-hatched diagram 2. vThe relative energy content of the various frequency components is shown by the horizontal distance between the vertical line 3, which is a zero reference line, and the line 4. The video signal band, as mentioned above, extends from .zero

filter I9 which passes bands I4 and I5 and supfrequency to approximately 806 kilocycles, as

indicated by the logarithmic frequency scale on the left-hand side of Fig. l. A corresponding scale appears on the right-hand side of Fig. 2. This video signal is impressed on a first modulator 3 to modulate a carrier frequency of 2376 kilocycles. Modulator 5 is specially designed to suppress practically completely all unmodulated carrier current. The important frequencies appearing in the output circuit of modulator 5 are a band of frequencies corresponding to the video signal represented by cross-hatched diagram 6 and upper and lower side-bands on either side of the l2376-ki1ocycle carrier represented by cross-hatched diagrams 'I and 9. The lower side-band extends from approximately 1570 kilocycles up to the carrier frequency 2376 and the upper side-band 9 extends from this carrier frequency up to approximately 3182 kilocycles.

The frequency components in the output of modulator 5 are impressed on a band-pass filter 95 which suppresses the video signal band 6 and a large portion of the upper side-band 9 and transmits the lower side-band 1 and a vestigial side-band. represented respectively by the crosshatched diagrams 9 and I0. The,l cut-oil? of lter 95 in the neighborhood of the 2376-ki1ocycle carrier is represented by line II extending to the left of reference line I2. The vestigial,

side-band extends to approximately 2450 kilocycles.

'Ihis lower side-band 9 and vestigial upper side-band I0 are? impressed on a second modulator I3 to modulate a carrier frequency of 2520 kilocycles. The important frequencies appearing in the output circuit of modulator I3 are the upper and lower side-bands corresponding to the bands 9 and I0 passed by the illter 95 andl the bands 9 and I0. The portion of the lower side-band coi-responding to band 9 is repregsented by cross-hatchedv diagram I4 and the I0 is represented by cross-hatched diagram I3, these two portions I4 and II lying, respectively, above and below 144 kilocycles, which is the component of the lower side-band of the second modulater I3 corresponding to the first carrier of 2376 kilocycles and the zero frequency component of the video signal 2. This lower side-band extends from approximately 70 kilocycles to 950 kilocycles. The portion of the upper side-band corresponding to band 9 is represented by crosshatched diagram I6 and the portion corresponding to the vestigial side-band I0 is represented by cross-hatched diagram I'I. This upper sideband extends from approximately 4089 kilocycles to 4944 kilocycles. modulator which transmits the modulating bands 9 and I0 represented by cross-hatched diagram 96 but suppresses the carrier of 2520 kilocycles.

The frequency components in the output of second modulator I3 are impressed on a low-pass presses any 2520-kilocycle second carrier leak, the modulating band 96 and the upper sidebands I9 andv I1. The transmitted bands I4 and I5 are represented by cross-hatched diagrams I9 and 20, respectively. The upper cut-oil of filter I8 is represented by line.2| extending to the left of zero reference line 22. These transmitted bands I9 and 20 extend from approximately 70 kilocycles to 950 kilocycles.

The transmitted bands I9 and 20 are amplined in amplier 23 an'd passed through a line predistortion network 24, another amplifier 25, an aperture equalizer 26, another amplifier 2'I and a terminal equalizer 29. 'Ihe combined atl tenuating characteristics of network 24 and -.equ|alizersY 26 andv 29 are represented by the 4b cross-hatched diagram band 30 -by suppressing un- 29 and the shape of the transmitted band is represented by the crosshatched diagram 30.

The frequency components appearing in the output of equalizer 28 are impressed upon a shaping high-pass lter 3I which narrows the all the frequency comlponents below 100 kilocycles ano shaping the components in the neighborhood of 144 kilocycles to eil'ect approximately one-half the effect reportion corresponding to the vestigial side-band quired by the principles disclosed in Nyquist Patent 1,748,186, supra. The frequency components corresponding to bands I9 and 20 appear in the out/put of filter 3I with relative energy values or band shape, as represented -by cross-hatched diagrams 32 and 33. The lower cut-oil of high-pass filter 3| is represented by line 34 extending to the left of zero reference line 35. These bands 32 and 33 are amplified in amplifier 36 varid transmitted to the line L which, preferably, is a coaxial c able provided with stiltable amplifiers and equalizers to transmit a wide band of frequencies with comparatively little distortion. v

It is seen from diagrams 32 and 33 that there is a band of transmitted frequencies on either side of 144 kilocycles. The band 33 above 144 kilocycles corresponds' Vto v the whole essential range of the video signals and is a complete side- =band, while the band 32 is a vestigial side-band. The component frequencies in the complete sideband, which are complementary to those in the vestigia] side-band, are attenuated about onehalf enough to compensate for the presence of the vestigia] side-band. Filter 3| also adjusts the phase of the transmitted frequency compo- Modumtor la is a balanced nents to compensate for phase distortions introduced by preceding cir-cuit elements.

Due to the transmission characteristics of .the

coaxial cable and included amplifiers', the band shape of the transmitted bands 32 and 33 is changed somewhat during transmission over the coaxial cable. The shape of these bands as they appear at the receiver terminal of coaxial cable L is represented by cross-hatched diagram 31.

This band 31 is Ipassed through high-pass fllter 38 and low-pass filter 39. These filters eliminate interfering frequencies outsideof the transmitted band 31 and further shape the -band in the neighborhod of 144 kilocycles. The shape of the band transmitted by filters 38 and 39 is represented by cross-hatched diagram 40 comprising the portion 4| above 144 kilocycles and the vestigial side-band portion 42 below 144 kilocycles. The cut-off of high-pass filter 38 is represented by line 43 extending to the left of zero reference line 44. The cut-off of low-pass-fllter 39 is represented by line 45, also extending to the left of zero referen-ce line 44. This band 40 extends from 100 kilocycles to approximately 950 kilocycles.y

The band 40 is amplified inA amplifier 48 and passed through an aperture equalizer and line restoring equalizer 41. The attenuating characteristics of equalizer 41 are represented by uncross-hatched diagram 48 and the shape of the transmitted band is represented by cross-hatched diagram 49 .com-posed of complete side-band 50 and a vestigial side-band The band 49 comprising bands 50 and 5| is impressed on a first demodulator 52-to modulate a carrier frequency of 2520 kilocycles. This demodulator 52 is designed to suppress the 2520- kilocycle cam-ier. The important frequencies appearing in the output of demodulator 52 are an upper and lower side-band and the modulating -band 49 represented by cross-hatched diagram 91. 'I'he lower side-band may be considered as comprising a band extending below 2376 kilocycles and a vestigia] side-band extending above 2376 kilocycles represented, respectively, 'by cross-hatched diagrams 53 and 54. The bands 53 and 54 correspond, respectively, to the portions 50 and 5| of the modulating band 48. These two bands extend from approximately 1570 kilocycles to 2420 kilocycles. The upper sideband is represented by cross-hatched diagram 55. This band extends from approximately 2620 kilocycles to 3470 kilocycles.

The frequency components in the output of demodulator 52 are impressed on a band-pass filter 58, an amplifier 51 and a 2520-ki1ocycle band elimination filter 58. The bandpass filter 58 passes the bands 53 and 54 which make up the lower side-band of demodulator 52 represented, respectively, by cross-hatched diagrams 98 and 99, and suppresses the upper side-band 55 and greatly attenuates any 2520-kilocycle carrier leak. The upper and lower cut-offs of filter 58 are represented by lines |00 and 59, respectively. The band elimination filter 58 completely suppresses for all practical purposes any remainder of the 2520-kilocyole carrier and effects a slight final shaping of the bands 80 and 8| in the neighborhood of the 2376kilocycle carrier in accordance with the principles of the Nyquist patent, supra. The attenuation characteristic of the band elimination filter 58 is represented by line 82. The final shaping effected by this filter 58 is inherent in its attenuation characteristic and is compensated for by under-shaping in high- Pass filter 38.

The finally shaped bands 80 and 8| are impressed on a second demodulator 83 to modulate a carrier frequency of 2376 kilocycles applied in proper phase relation to the frequencies of the modulating bands 80 and 8| to produce a signal current according to the Nyquist principle. The important frequencies appearing in the output of demodulator 83 are the video signal which is substantially identical with that impressed on the iirst modulator 5 except that it is equalized to compensate for aperture distortion in the scanning devices, represented by the cross-hatched diagram 84, a band of frequencies corresponding to the modulating bands 80 and 8| represented by cross-hatched diagram 85, and an upper sideband represented by cross-hatched diagram 88.

The frequencies in the video signal band 84 ex tend from zero frequency to about 806 kilocycles. The frequencies in band 85 extend from about 1570 kilocycles to 2420 kilocycles. The frequencies in the upper side-band 88 extend from approximately 3946 kilocycles to 4796 kilocycles. 'I'he second demodulator 83 is specially designed to suppress the carrier frequency or 2376 kilocycles completely for all practical purposes.

'I'he frequency components in the output of I second demodulator 83 are impressed on a lowpass filter which passes only the video signal band represented by cross-hatched diagram 88. This video signal band 88 is applied to cathode ray tube 89 to control the brightness of the scanning spot on the fluorescent screen. This video signal band 88 has a. form suitablefor compensating for the finite size of spot in the cathode ray tube, this compensating shape resulting from the operation of the aperture equalizer 41.

The 2376-kilocycle and 2520-ki1ocycle carrier frequencies accurately spaced are derived from an accurately controlled carrier supply source 18. From this source 10 a 72-kilocycle frequency is ilrst obtained. This 72-kilocycle standard frequency is passed through a purifying crystal band-pass filter 1| and impressed on a harmonic generator 12. The thirty-third harmonic or 2376- kilocycle carrier is selected from the output of harmonic generator 12, passed through crystal band-pass filter 13and applied to the first modulator 5. kilocycle carrier is also selected from the output of harmonic generator 12, passed through crystal band-pass lter 14 and applied to the second modulator I3.

A 5.76-kilocycle frequency for controlling the sweep circuits of the cathode ray tube 68 is obtained directly from a synchronizing impulse producer 15 associated with the scanning disc. 'I'his 5.76-kilocycle frequency from synchronizing Impulse producer 15 is used to modulate a portion of the standard 72-kilocycle frequency from iilter 1| in modulator 18. The 72-kilocycle carrier and the lower side-band frequency of 66.24 kilocycles resulting from the modulation are selected by double band crystal filter |15, which frequencies are impressed on the coaxial cable L at the transmitting terminal and transmitted to the receiver.

At the receiving terminal the 72-kilocycle carrier and the 66.24-kilocycle side-band frequency are selected from the coaxial cable L by the double band crystal filter 11 and impressed on demodulator 18, from the output of which the 72-kilocycle frequency is selected by crystal bandpass filter 18, and the lower side-band frequency The thirty-fifth harmonic or 2520- of 5.76 kilocycles is selected by the crystal bandpass lter 80. 'Ihe 5.76-kilocyc1e frequency is used to control the sweep circuit generator 9| which furnishes the sweep currents for cathode ray tube 69. The 72-kilocycle frequency from lter 19 ls impressed on harmonic generator 82. From this harmonic generator 82 the 2520-ki1ocycle carrier is selected by crystabband-pass lter 83 and applied to the first /demodulator 52 and the 2376-kilocycle carrier is selected by crystal band lter 84 and applied to the second demodulator 63 in proper phase. The phase may be adjusted manually, the proper adjustment being indicated by the appearance of the produced image on the screen of the cathode ray tube 69.

The program frequency band is transmitted las modulations of an 84kilocycle carrier from carrier source 10. The program frequency band is derived from the sound track on the lm in sound reproducer 85. The 84-kilocycle program carrier frequency is passed through crystal bandpass filter 86 and, together with the program frequency `band from sound reproducer 85, is impressed on modulator 81. The lower side-band resulting from the action of modulator 81 is selected by band-pass filter 88 and impressed on the coaxial cable L at the transmitting termi-- nal and transmitted to the receiver.

At the receiving terminal the program sideband is selected by band-pass filter 89 and an 84-kilocycle carrier selected by band-pass filter 9| from carrier supply source 90 together are impressed on demodulator 92. The program frequency band is selected from the output of demodulator 92 by band-pass filter 93 and applied to 1oud-speaker94 for program reproduction.

The relative positions of the program band, the 72-kilocycle standard frequency, and the synchronizing 66.24-kilocycle side-band frequency with respect to the transmitted television band 31 on the coaxial cable L are shown below the cross-hatched diagram 31 in Fig. 2. Two pilot channels are also provided, as indicated in Fig. 2, at 1024 kilocycles and 60 kilocycles for controlling the gains of the coaxial cable amplifiers.

Referring now to Figs. 3 and 4, the circuits of the rst modulator 5, band-pass lter 95, second modulator I3 and the connection to the first modulator 5 of the light sensitive electric device I in the form of a photoelectric electron multiplier Will be described. 'I'he coaxial cable I'II at the right side of Fig. 3 and the coaxial cable |12 at the left side of Fig. 4 are portions of the same.

cable.

As hereinbefore described the light signals coming from the scanning disc fall upon the photoelectric cathode of the electron multiplier |I which transforms them into video signals which are transmitted from the multiplier IOI by means of a section of double shielded coaxial cable |02 to the rst modulator 5. In the input circuit of the rst modulator the condensers |03, |04 and |05 and coil |06 form a low-pass network which when terminated in the resistance |01 nominally 1,000 ohms present a 1000- ohm impedance to the terminals of the electron multiplier at video frequencies. 'I'he vacuum tubes |08 and |09 in the first modulator 5 are so-called space charge pentodes. After the cathode the irst electrode is a space charge grid, hereafter known as the net electrode. The next electrode is the ordinary control grid, the third signal is placed on the control grid of vacuum tube |08. The 2376-kilocycle carrier frequency a screen grid, and the last the plate. 'Ihe video comes in on a coaxial cable I|0 to a potentiometer ||I and sis applied across the net electrodes of both tube |06 and |09. This carrier circuit is bridged by a resistance ||2 in series with a thermocouple ||3 for the purpose of reading the carrier frequency voltage applied. The positive direct current voltage on the net 'electrodes is supplied by the battery II4 through the filter consisting of condensers H5 and |I6 and coils I|'| and ||6. The passive tube |09 serves the sole purpose of balancing out the carrier frequency in the output of the modulator 5. Condensers |I9 and |20 and resistance |2| are adjusted to make the impedance facing the control grid ofthe passive tube |09 equal to that facing the control grid of the active tube |08 at carrier frequencies. The negative control grid bias of the passive tube |09 is furnished by battery |22 and may be adjusted by means of potentiometer |23 to make the carrier frequency transmission of the passive tube |09 equal to that of the active tube |08. The negative bias on the control grid of the active tube |08 is |24. The positive screen bias is furnished by the tap on battery |25 through coil |26 and the screen grids are shunted by condenser |21. The two plates are shunted by reslstances |28 and |28 and connected tc an output network consisting of condensers |30 and |3| and transformer |32. Condenser |30 really consists of two condensers, one of which subtracts capacity from one plate circuit as it is added to the other. The law of variation governing these two condensers is such that the total series capacitative impedance across both plates is at 'all times kept constant. Coil |33 and condenser |34 form a circuit tuned to the second harmonic of the 2376-kilocycle carrier frequency to aid in its suppression. It will be seen that there are two controls affecting the balance of carrier frequency at the outthe way down to direct current, any direct current coming from the electron multiplier will unbalance ythe modulator causing unmodulated carrier frequency to ow in its output which is exactly proportional to the direct current in the input circuit. This makes it possible for information concerning the average background of the picture to be transmitted. The band lter |35 selects the lower side-band of the carrier and a small portion of the upper sideband. The input network of the second modulator I3 consisting of transformer |36, condensers |31 and |36 and resistances |39 and |40 is similar to the output network of the rst modulator 5. The second modulator |3 is a conventional balanced modulator employing power pentodes |16 and |11. The signal frequency is applied to the control grids of the two tubes in series and the carrier frequency is applied to the control grids of the two tubes in parallel. The 2520-kilocycle carrier frequency is applied through a parallel tuned circuit consisting of condenser |4I and coil |42 across the resistance |43. Potentiometer furnished by battery |44V allows a balance to be obtained against the carrier frequency of the second modulator |3 in its output circuit. Plate and screen potentials for the tubes are obtained from a 250-volt supply and applied to the screens through coil |45 and to the plates through coil |46. Condensers |41 and |48 in conjunction with coils |45 and |46 act as power filters for supplying the screen and plate potentials. Grid bias is obtained by the drop acrossresistance |53 and the parts of'pctentiometer |44. The output circuit consists of transformer |49 shunted by resistances |50 and |I on its high impedance winding and shunted by resistance |52 on the low impedance winding.

In order to fully disclose the method of construction of the illustrative embodiment of the invention hereinbefore described, the values of certain of the circuit elements will now be given. Referring particularly to Fig. 3, condenser |03 has a capacitance of 184.9 micro-microfarads less the inherent capacitance of the section of coaxial cable |02 and the electron multiplier |0|.

4 The condenser |04 has a capacitance of 73.95

micro-microfarads. Condenser |05 has a capacitance of 40.19 micro-microfarads less the inherent capacitance of active tube 8. Coil |06 has an inductance of' 125.8 microhenries. Resistance |01 has a value of 1,000 ohms. Vacuum tubes |03 and |09 are identical and preferably are Western Electric Code No. 75751? vacuum tubes. Potentiometer has a resistance of 400 ohms and resistance ||2 has a value of 1,800 ohms. The thermocouple ||3 has a resistance of 500 ohms.

Battery ||4 is of 10.5 volts. Condensers ||5 and ||3 each have a capacitance of .03 microfarad. Coils ||1 and ||8 each have an inductance of 1 millihem'y. Condenser ||9 is adjustable and has a nominal capacitance of 50 micro-microfarads. Condenser |20 has a capacitance of 20 micromicro-farads. Resistance |2| has.aI value of 1,000 ohms. Battery |22 is of 9 volts. Battery |24 is of 1.5 volts. The total voltage of battery |25 is of 250 volts and the lower portion of 160 volts. Coil |26 has an inductance of 1 millihenry. Condenser |21 has a capacitance of .7 microfarad. Resistances |28 and |29 each have a value of 3,000 ohms. The left-hand winding oftransformer |32 has an inductance of 243 microhenries and right-hand windings in parallel have an inductance of 20.2 micro-henries. Condensers |30 and. |3| are adjustable and in paralvlel with the tube capacities are set to tune the left side windings of transformer |32 to 1,862 kilocycles. Coil |33 has an inductance of 100 microhenries and condenser |34 hasa capacitance 'of 25 micro-microfarads, the condenser |34 being adjusted to form a tuned circuit with coil ||3 tuned to a frequency of 4,752 kilocycles.

Referring now to Fig. 4, the band filter |35 comprises two series elements in the form of parallel tuned circuits |60 and |6| and a shunt element in the form of a parallel tuned circuit |32. Tuned circuit |60 consists of a coil |63 having an inductance of 149.6 microhenries and a condenser |64 set to tune this circuit |60 to a frequency of 1,000 kilocycles. Tuned circuit |6| consists of a coil |65 having an inductance of 21.0 microhenries and a condenser |66 set to tune this circuit |6| to a frequency of 2,625 kilocycles. Tuned circuit |62 consists of a coil |61 having an inductance of 55.3 microhenries and a condenser |68 set to tune circuit |62 to a frequency of 2,350 kilocycles. Condenser |69 has a capacitance of 344.8 micro-microfarads and contube capacities, are set to form a tuned circuit with the right-hand winding of transformer |33 tuned to a frequency of 1,790 kilocycles. Resistances |39 and |40 each have a value of 3.000 ohms. Condenser |4| and inductance coil |42 are adjusted to be antiresonant at 2,520 kilocycles. Resistance |43 has a value of 100 ohms. Potentiometer |44 has a resistance of 150 ohms while the condensers connected to either terminal of resistance |44 each has a capacitance of .7 microfarad and the resistance connected to the variable contact of resistance |44 has a value oi' 500 ohms. Each of resistances |50 and |5| has a value of 2,140 ohms. Resistance |52 has a resistance oi' 260 ohms. Tubes |15 and1|11 are identical and preferably are Western Electric Code No. 7629 vacuum tubes.

The high-pass filters 3| and 33 are constructed each with amplitude attenuating sections and phase adjusting sections according to well-known filter design principles. Sufficient sections of each kind are used to produce the'band shapes described hereinbefore.

What is claimed is:

-1. An electro-optical image producing system comprising means to produce a video signal having frequency components extending from a very low frequency to several hundred kilocycles, means to produce a control frequency lying within the range of frequencies constituting the video signal. means to produce two carrier frequencies which constitute adjacent odd harmonics of said v control frequency, means to modulate the lower of said carrier frequencies with said video signals, means to select and transmit only the lower side-band and vestigial upper side-band resulting from said modulation, means to modulate said higher carrier with said transmitted sidebands, means to select only the lower side-band resulting from said second modulation. and means to transmit said last selected side-band and produce a television image therefrom.

2. A television system comprising means to produce a video signal having components extending from a very low frequency to several hundred kilocycles, a first modulator comprising an active tube and a passive tube each having a cathode, a net electrode, a control grid, a screen grid and a plate, means to impress said video signal on the cathode and control grid of said active tube only, means to impress a nrst carrier wave on the cathodes and net electrodes of both said tubes in parallel, means to impress a direct current potential on the screen grids of both said tubes in parallel, means to impress direct current biasing potentials on the control grids ci' both said tubes, means to independently adjust the biasing potential on the control grid of said passive tubeand the alternating current impedance between the cathodes and the plates of both said tubes to completely suppress any component of unmodulated carrier current in the output circuit of said modulator, a band pass filter transmitting the entire lower side-band and a portion of the upper side-band of frequencies received from said first modulator and suppressing the rest of said upper side-band, a second modulator comprising means to modulate a second carrier current with the side-band fre-l quencies transmitted by said band pass lter and suppress said second carrier, means to produce a second carrier of frequency slightly higher than the highest upper side-band frequency transmitted by said band pass lter, and means to transmit a. portion only of the lower side-band produced by saidsecond modulator and suppress the other sideeband and differently attenuate the transmitted side-band on either side of the sideband frequency corresponding to the frequency of said rst-mentioned carrier current.

CHARLES L. WEIS, JR. 

